It has long been desirable to capture radiation, particularly visible light, to convert it directly into electrical energy through the utilization of photovoltaic cells. Many types of photovoltaic cells, often referred to as solar cells, have been considered and constructed. For example, single-crystal cells have been produced, as well as those produced from gallium arsenide and other similar materials. In addition, thin film cells have been fabricated from microcrystalline, amorphous, compound or semiconductor material other than single crystal semiconductor material, deposited in situ upon a substrate by chemical vapor deposition, sputtering or other similar means. In use, these cells are assembled in photovoltaic panels and modules which must withstand the rigors of the environment and handling in commerce.
As used herein, the term photovoltaic panel refers to a combination of a sheet of transparent material or other lamina, an array or group of photovoltaic cells interconnected to provide an output of electrical energy, and any backing sheet or material, which forms a device capable of transforming incident radiation to electrical current. Such panels are traditionally comprised of a transparent front or radiation-facing sheet such as a glass or transparent polymer, laminated with layers of transparent conductors, photovoltaic materials, cell-connecting circuits, metals and other lamina which together comprise an operative photovoltaic panel. Thus, photovoltaic panels have traditionally included a sheet of glass or other rigid transparent material to protect the photovoltaic cell, and a back sheet of steel or aluminum metal or foil, with the various lamina being bonded together by a dielectric layer of plasticized polyvinyl butyryl or ethylene vinylacetate In instances where a totally transparent photovoltaic panel is desired, such as in a solar cell which serves as an automobile sun roof, a front and back sheet both of rigid transparent material is employed.
After the initial assembly of the laminates which comprise the photovoltaic panel, the edges of the panel have traditionally been smoothed to provide a flush edge surface, and sealed with a non-conductive varnish followed by one or more layers of polyester and/or polyurethane tape. After this sealing of the edges, the panel is enclosed in a peripheral frame of aluminum, steel, molded polymer or other rigid frame material This method of sealing and framing the periphery of the panel has been necessary to isolate the solar cell from the environment, and to provide a frame for the strengthening of the panel and to provide a border to permit ease of handling and the attachment of connector boxes and the like for attachment of the photovoltaic cell to an external electrical circuit. For example, a solar panel with a hardened foil back layer sandwiched between polyvinyl fluoride resin sheets, and framed in rigid peripheral framing is shown in U.S. Pat. No. 4,401,839. This combination of a photovoltaic panel with the frame, sealing means, connection means and ancillary supporting structures is referred to herein as a photovoltaic module.
While existing methods for the production and framing of photovoltaic panels have provided significant improvements in solar cell technology over the years, it has been a desideratum to simplify the lamination and manufacture of such panels and provide for a stronger module and a more perfect seal to protect the edges or back panel of photovoltaic panels. For example, the lamination steps have previously required a considerable expenditure of labor, and the metal backing sheets previously used to protect the back of the panel may allow electrical leakage which turns a photovoltaic cell into a capacitor.
A thin film photovoltaic module produced by the method of the invention has a panel with front and back sides and edges forming a perimeter and at least one photovoltaic cell capable of converting radiation incident on the front of the panel to electrical energy, the panel being partially encapsulated in a unitary, reaction-injected molded elastomer which forms a seal against a portion of the front side of the panel bordering the perimeter, and continues around the perimeter and seals against at least a portion of the back side. In one embodiment, the module fabricated by the method of the invention further comprises means for establishing external electrical connection to the photovoltaic cell, including an internal portion electrically connected to the cell and an external portion extending from the panel, and the unitary elastomer further encapsulates at least the internal portion of such connecting means.
The method of the invention involves placing a thin film photovoltaic panel in a mold having interior walls which cooperate with a predetermined portion of the module to define a cavity encompassing the perimeter and at least a portion of the front and back of the panel, introducing a flowable reaction injection molding material into the cavity and curing the material to form the casing. For example, the panel could be placed in a mold including two cooperating mold sections having surfaces defining a chamber for receiving the panel, with one of the mold sections having seal means positioned to be adjacent the periphery of the front side of the panel to support the panel within the panel receiving chamber and seal the portion of the panel located interiorly of the seal means against the influx of fluid. The facing surfaces of the mold sections located exteriorly of the seal means are provided with a casing shaping portion which cooperates with the seal means and the predetermined portion of the panel to define a cavity when the mold sections are in contacting relationship.
The mold also includes inlet means for introducing the flowable reaction injection molding material into the cavity when the mold sections are contacting to form a closed mold. The casing shaping portion of the mold may also define a cavity portion which includes at least the internal portion of the connecting means of the panel so that a portion of the connecting means is imbedded in the cured reaction injection molding material. Preferably, the reaction injection molding elastomer encapsulates the back and perimeter edges of the panel and forms a seal against a portion of the front side of the panel bordering the perimeter. If desired, a stiffening structure such as metal, fibrous or polymeric sheets or girders may be included within the elastomer to add to the rigidity of the photovoltaic module.
In a preferred embodiment, the module produced by the method of the invention comprises a first sheet having a first radiation-incident side and having a photovoltaic cell formed thereon; a second backing sheet, preferably having a comparable modulus of thermal expansion, adjacent and spaced from a back side of the first sheet; and a unitary, reaction injection molded elastomer disposed between the back side of the first sheet and the second, the elastomer partially encapsulating the first sheet, forming a seal against a portion of the front side of the first sheet bordering the perimeter, continuing around the perimeter of the first sheet and sealing against at least a portion of the second sheet. The term unitary elastomer, as used herein, refers to a one-piece elastomer, one that is formed by a single injection of an elastomer-forming material as described.